Monte Carlo model for electron degradation in methane gas

Abstract

We present a Monte Carlo model for degradation of 1–10,000 eV electrons in an atmosphere of methane. The electron impact cross sections for CH₄ are compiled and analytical representations of these cross sections are used as input to the model. “Yield spectra”, which provides information about the number of inelastic events that have taken place in each energy bin, is used to calculate the yield (or population) of various inelastic processes. The numerical yield spectra, obtained from the Monte Carlo simulations, is represented analytically, thus generating the Analytical Yield Spectra (AYS). AYS is employed to obtain the mean energy per ion pair and efficiencies of various inelastic processes. Mean energy per ion pair for neutral CH₄ is found to be 26 (27.8) eV at 10 (0.1) keV. Efficiency calculation showed that ionization is the dominant process at energies >50eV, for which more than 50% of the incident electron energy is used. Above 25 eV, dissociation has an efficiency of ~27%~27%. Below 10 eV, vibrational excitation dominates. Contribution of emission is around 1.2% at 10 keV. Efficiency of attachment process is ~0.1%~0.1% at 8 eV and efficiency falls down to negligibly small values at energies greater than 15 eV. The efficiencies can be used to calculate volume production rate in planetary atmospheres by folding with electron production rate and integrating over energy.